1. Field of the Invention
The subject invention relates to improving the image on a display of a color television receiver as seen by the viewer by giving more true to life color.
2. Description of the Related Art
The current in a cathode ray tube (CRT), like other triode vacuum tubes, is a non-linear function of drive voltage. A power function fits the data quite well over several decades: EQU I=k*V.sup..gamma. ( 1)
where I is the cathode or anode current, V is the CRT drive voltage, k is the transconductance, and .gamma. is the gamma value depending on the detailed design of the electron gun in the CRT.
Conventionally, the value of .gamma. for grid drive CRT's is taken to be 2.2, and for cathode drive CRT's, .gamma. is taken to be 2.8 In addition, it is normally assumed that .gamma. for each of the three guns in a color CRT is the same. For direct view CRT's, it is also normally assumed that the phosphors are linear with current.
While it has been found that these assumptions are not accurate, the errors introduced are to such a small extent that if only luminance and monochrome images are examined, it will make little difference. However, when chromaticities of desaturated colors are examined, the errors introduced can be large. One manifestation of this is the difficulty in getting satisfactory skin tones on a CRT display.
However, these assumptions are built into the television industry. Receiver manufacturers expect gamma correction and color encoding to be controlled by broadcasters and other video sources to match the CRT characteristics specified by the NTSC standard. However, the CRT defined by the NTSC standard bears little relationship, in terms of .gamma. or chromaticity, to CRT's currently in production. Receiver manufacturers add decoding circuits to partially correct for the color differences, but do not correct for the gamma differences. This decoding is done by the receivers to display the most pleasing picture on the screen, which is not always the picture with the most accurate color rendition. Color errors of one type are used to balance errors of a different type, leading to improvement in one portion of the color gamut and a degradation in another region.
Studies have identified three major sources of color error:
(1) Cathode bias offset from the correct value for the particular tube in a television receiver; PA1 (2) Differential .gamma. between the red, green and blue colors (R, G and B) in a television receiver, these gamma differences arising either from differences between the red, green and blue guns in a CRT, or from phosphor non-linearities; and PA1 (3) Gamma difference between the broadcast signal with a nominal .gamma. of 2.2 and the receiver .gamma..
In regard to error (3), measured receiver .gamma. values were in the range of 2.1-2.7.
In correcting these errors, manufacturers deliberately introduce errors of type (1) to partially cancel the errors of type (2). If type (1) errors are corrected without correcting type (2) errors, the accuracy of the color rendition is actually reduced. Therefore, it is necessary to correct both of these type of errors simultaneously in order to improve the color rendition. As such, equation (1) may be modified as follows: EQU I.sub.k =k(V.sub.s +V.sub.ERROR).gamma. (2)
where V.sub.s is the signal voltage, V.sub.ERROR represents an error in setting the cathode cutoff value, or a deliberate mis-setting of the cutoff, and .gamma. is the gamma of each gun. If V.sub.ERROR is positive, the gun produces some current at 0 V.sub.s. If V.sub.ERROR is negative, the gun produces 0 current for all V.sub.s .ltoreq.V.sub.ERROR The "brightness" control on the CRT receiver allows the consumer to offset all three guns with the same V.sub.ERROR.
With regard to type (3) errors, studies have shown that consumer actually prefer pictures when there is a positive type (3) error, that is, when .gamma..sub.RECEIVER &gt;.gamma..sub.BROADCAST, or .gamma..sub.RECEIVER &gt;2.2 Under these conditions, the colors on the screen tend to be more saturated, i.e., more "vivid".